This invention relates to the removal of noxious substances from exhaust streams from semiconductor manufacturing processes. Many substances used in semiconductor device manufacturing, and which are extracted from a chamber in which such manufacturing takes place normally by means of a vacuum pump, are toxic or environmentally harmful and must therefore be removed or scrubbed from the exhaust gas stream to render it harmless before its release in to the atmosphere.
A number of different types of wet or dry chemical scrubbing reactors have been proposed and numerous are commercially employed in the semiconductor industry.
For example in U.S. Pat. Nos. 5,213,767 and 5,417,934, the contents of which are incorporated herein by reference there is disclosed a dry chemical reactor comprising a packed tube, for example 1.5 m in length, of granular substances heated to 500xc2x0 C. and through which the exhaust stream is directed including in particular a first stage of silicon (with an optional addition of copper when the exhaust stream contains nitrogen trifluoride (NF3)) and a second stage of calcium oxide commonly in the form of lime. Such a reactor sold as the GRC(trademark) reactor by BOC Edwards has met with considerable commercial success for the scrubbing of such toxic substances.
A relatively new substance used in metal oxide chemical vapour deposition (MOCVD) semiconductor processing is 1,1,1,5,5,5-hexafluoropentane-2,4,dionato copper (1) trimethylvinylsilane sold under the trade name xe2x80x9cCupraselectxe2x80x9d. Use of this organo-metallic substance in MOCVD processing produces trimethylvinylsilane (TMVS) as a by-product which must be scrubbed from exhaust stream exhaust from the processing chamber.
It is known that a certain abatement of TMVS may be effected by thermal decomposition methods. Such methods include the use of the GRC reactor described above which generally would involve passing the TMVS vapour in an exhaust stream diluted in nitrogen gas through the heated silicon and calcium oxide granules. However, it has been found that such thermal decomposition methods can remove only eighty or ninety per cent of the TMVS in the exhaust stream.
In view of the strongest regulations regarding exhaust emissions adopted by governmental authorities and by the semiconductor companies themselves, however, threshold limiting values (TLVs) are set for the different exhaust gas species. Although no specific TLV has yet been set for TMVS, it will undoubtedly be much more onerous than the levels of abatement so far achieved with thermal decomposition methods, for example no more than 10 parts per million (ppm) TMVS emitted to the atmosphere.
There is, therefore, a need for a process in which TMVS is abated to a much greater extent than is currently available.
In accordance with the invention, there is provided a process for the abatement of trimethylvinylsilane (TMVS) by contacting a gas stream containing TMVS with copper (II) oxide (CuO) and/or manganese oxide (MnO2) in the presence of sufficient oxygen to prevent reduction of the oxides.
Generally, the oxide temperature will be at least room temperature for example at least 20xc2x0 C. and will preferably be at least 100xc2x0 C.
It is thought that the copper oxide and/or manganese oxide catalyses the oxidation of TMVS in accordance with the following general equation:
(CH3)3SiCH=CH2+902xe2x86x925CO2+6H2O+SiO2
although the invention does not depend on this particular reaction occurring.
Preferably both copper(II) oxide or manganese oxide are present in the process of the invention as an admixed mixture. Advantageously at least 10% of each oxide is present on a mole fraction basis, most advantageously at least 30% of each oxide is present again as a mole fraction basis, for example 50% of each or 60% manganese oxide/40% copper oxide.
A suitable copper oxide and manganese dioxide admixture can be prepared by adding copper(II) sulphate solution to a sample containing a fine suspension of carefully prepared manganese dioxide, mixed with sodium carbonate. An intimate mixture of manganese dioxide and basic copper carbonate results. Heating the precipitated carbonate mixture produces a truly admixed copper oxide and manganese oxide suitable for use in the invention.
A commercial catalyst comprising about 50% of copper oxide and 50% of manganese oxide, on a mole fraction basis, is available under the trade name xe2x80x9cMoleculitexe2x80x9d from Molecular Products Limited (UK).
It has been found that the use of a mixed copper oxide/manganese oxide catalyst is surprisingly beneficial in TMVS abatement when operated at the high temperatures required in the process of the invention.
It is preferable for all the Cupraselect which may not have been reacted in the semiconductor processing chamber and which is therefore present in the exhaust stream to be itself decomposed prior to the process of the invention. This can be accomplished by first completely cracking any unreacted Cupraselect on a heated surface at a temperature of 150xc2x0 C. to 350xc2x0 C. This action will liberate TMVS and bis-(1,1,1,5,5,5-hexafluoropentane-2,4,dionato)-copper, (or bis-hexafluoroacetylacetonato-copper(II)), the latter of which is commonly known as xe2x80x9cCu(II)(Hfac)xe2x80x9d2.
The positioning of apparatus for carrying out the invention within an exhaust line from, for example, a semiconductor processing chamber can be effected to suit the particular circumstances pertaining in the line/chamber.
In preferred embodiments, the unreacted Cupraselect from a semiconductor processing operation will deposit copper (metal) on the inner surfaces of the hot exhaust line or of hot valves and/hot pumps used therein. Preferred embodiments of the invention therefore position a hot cracker in the process exhaust line from the reaction chamber, and before (upstream of) the vacuum pump, to decompose all the xe2x80x9cCupraselectxe2x80x9d before it can decompose in the pumping mechanism. The resulting by-products, including TMVS are thereafter pumped through the hot mechanism of the pump, before the Cu(II)(Hfac)2 is condensed in an air cooled condenser and the TMVS is passed to the process of the invention for treatment. In such embodiments, apparatus for carrying out the process of the invention can advantageously be positioned downstream of the vacuum pump or pumps in particular.
In its simplest form, the process of the invention may be carried out by placing copper oxide and manganese oxide particles in a heated tube, for example stainless steel, at a temperature of at least room temperature (20xc2x0 C.) and preferably above 100xc2x0 C. and passing the TMVS containing exhaust stream and a source of oxygen through the tube.
The source of oxygen may be oxygen or oxygen-containing gas mixtures including air. The oxide mixture may be simply the oxide particles themselves or may be mixed with a suitable inert carrier material.
In preferred embodiments, the process may be carried out in conjunction with a dry scrubber, for example of the GRC reactor type described herein. In such embodiments, the oxide mixture may be admixed with the dry scrubber reagents. In the case of a GRC reactor in particular, the copper oxide/manganese oxide particle mixture can advantageously be added at the exhaust end of the calcium oxide (second) stage, ideally by admixing with the calcium oxide to a depth of, for example 100 mm.